Controlling Phytophthora blight of pepper in Guizhou Province of China using Stellera chamaejasme extracts and synthetic chemical fungicides

Phytophthora blight of pepper, caused by Phytophthora capsici Leonian, is a destructive disease in pepper production. Extracts of the plant species Stellera chamaejasme Linn, previously known in China as a source of herbal medicine, were also used in former years as a toxin against aphids, mites, and plant-pathogenic fungi. Extracts of S. chamaejasme (SC) and seven commercial fungicides were investigated for their inhibition of mycelial growth and germination of cysts of the pathogen in vitro. The SC alone, infinito (fluopicolide + propamocab + hydrochloride), fluazinam, dimethomorph, and their mixtures with SC strongly inhibited both mycelial growth and germination of cysts of P. capsici. Tests were conducted in a commercial field in Fenggang County, Zunyi City, Guizhou Province, China, in 2019, 2020, 2021, and 2022 to validate these results for growers. A soil application (7 days before transplanting) and three foliar sprays at 7, 10, or 15 days were tested. One soil application of SC followed by three successive foliar sprays of infinito, fluazinam, and dimethomorph at 7- or 10-day intervals, as well as tank-mixed applications of these synthetic fungicides with SC at 15-day intervals, limited Phytophthora blight incidence to < 10% on the non-treated control treatment. Disease incidence was limited to < 3% to < 5% when alternating SC, SC + infinito, SC + fluazinam, and SC + dimethomorph three times at 7-, 10- or 15-day intervals, plus one soil application of SC. When treatments were applied from early April to May, disease incidence was < 10% in the non-treated control during June and July. The optimal concentration of SC was determined to be 0.25 g/L, which effectively controlled Phytophthora blight and protected marketable yield. SC also significantly (P < 0.05) outperformed treatments in a nearby commercial pepper field that relied on applications of a single synthetic chemical fungicide. The findings of this study provide a foundation for guiding growers to implement an efficient and environmentally safe spray program against Phytophthora blight of pepper in Guizhou Province.